The invention relates generally to the assessment of the condition of dynamoelectric machines. More particularly, the invention relates to a system and method for measuring or determining the tightness of wedges in dynamoelectric machines, particularly electric generators.
Dynamoelectric machines such as electric generators include a rotor and a stator. Rotors are generally constructed from a steel forging and include a number of slots that run the length of the rotor. Rotors are electrically wound by placing conductors referred to as rotor windings into the slots of the rotor.
Stators are generally constructed from a number of stacked, metal laminations. Stators also include slots, which run the length of the stator. Stators are electrically wound by placing conductors known as stator coils into the armature slots of the stator.
Conventional stator coils are frequently held in place in stator slots using a retention assembly such as a stator wedge assembly including a stator wedge, a top retaining ripple spring, and one or more shims. In this configuration, a stator coil is placed into an armature slot, a shim is placed above the stator coil, a top ripple spring is placed above the shim, and a stator wedge having a beveled edge is driven into a groove near the head of the armature slot, securing the stator coil, the shim, and the top ripple spring. The top ripple spring provides compressive force to keep the stator coils held firmly in the armature slot.
Over time and during use of the dynamoelectric machine, stator wedges may become loose. If a stator wedge becomes loose, it can permit a stator coil to vibrate, which can cause catastrophic failure in an electric generator. In order to avoid such vibration, it is desirable to periodically inspect the tightness of the wedge assembly. Such inspections present a challenge, because the wedge assembly is difficult to access within a generator.
There are a number of conventional approaches to inspecting the compression of wedge assemblies. One approach involves manually tapping the stator wedges. This method includes tapping the stator wedge and feeling or listening to the response therefrom to make a judgement on stator wedge tightness. This method depends on the experience of the technician doing the measurement and is extremely subjective.
Another approach involves measuring the depth of the surface of ripple springs through pre-formed test holes in the wedge. This method uses a depth gauge to take measurements through pre-formed test holes, and is time consuming and only possible in generators having stator wedges with pre-formed test holes. Many generators do not have such pre-formed test holes. In order to use this method on existing generators without test holes in the stator wedges, the units must be rewound using wedges with access holes.